This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-074684, filed Mar. 15, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical sampling waveform measuring apparatus and more particularly to an optical sampling waveform measuring apparatus aiming at a wider band when measuring an optical pulse waveform of an optical signal used for an optical communication and the like with a sum frequency generation light (SFG light).
2. Description of the Related Art
Generally, when constructing a new optical communication system, manufacturing a new optical transmission apparatus or inspecting such optical communication system and optical transmission apparatus periodically, it is important to measure a pulse waveform of a digital optical signal to be transmitted received in order to grasp the quality of the optical communication.
In recent years, transmission velocity of information in optical communication has been increased and currently, high-speed optical transmission of 10 Gbit/s or more has been planned.
Jpn. Pat. Appln. KOKOKU Publication No. 6-63869 has disclosed an optical sampling waveform measuring apparatus for measuring an optical pulse waveform of high-speed optical signals of more than 10 Gbit/s with the sum frequency generation light.
FIGS. 7A, 7B and 7C and FIGS. 8A and 8B explain the measuring principle of the optical sampling waveform measuring apparatus disclosed in this Jpn. Pat. Appln. KOKOKU Publication No. 6-63869.
For example, if a measuring object light xe2x80x9caxe2x80x9d having repetition frequency xe2x80x9cfxe2x80x9d of the pulse waveform of measurement object and a sampling light xe2x80x9cbxe2x80x9d having a pulse width by far narrower than the pulse width of the measuring object light xe2x80x9caxe2x80x9d and having a repetition frequency (fxe2x88x92xcex94f) slightly lower than the repetition frequency xe2x80x9cfxe2x80x9d of the measuring object light xe2x80x9caxe2x80x9d are entered into a nonlinear optical material 1 allowing type 2 phase matching to the measuring object light xe2x80x9caxe2x80x9d and the sampling light xe2x80x9cbxe2x80x9d simultaneously, only when the two lights xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d overlap each other at the same time, a sum frequency light xe2x80x9ccxe2x80x9d proportional to a product of the intensities of these two lights xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d is outputted from the nonlinear optical material 1.
Because the repetition frequency of this sum frequency light xe2x80x9ccxe2x80x9d is the repetition frequency (fxe2x88x92xcex94f) of the sampling light xe2x80x9cbxe2x80x9d, response velocity of a photo-electric converter which converts this sum frequency light xe2x80x9ccxe2x80x9d to an electric signal only has to be higher than the repetition frequency (fxe2x88x92xcex94f).
Further, because the time resolution of this photo-electric converter is determined depending on the pulse width of the sampling light xe2x80x9cbxe2x80x9d, if the envelope waveform of this electric signal is obtained after the sum frequency light xe2x80x9ccxe2x80x9d is converted to an electric signal by means of this photo-electric converter, the shape of the envelope of this electric signal is an optical pulse waveform xe2x80x9cexe2x80x9d of the measuring object light xe2x80x9caxe2x80x9d enlarged on time axis.
Next, the sum frequency light and phase matching will be described below.
If the measuring object light xe2x80x9caxe2x80x9d having an angular frequency xcfx89D and the sampling light xe2x80x9cbxe2x80x9d having an angular frequency xcfx89S are entered into one face of the nonlinear optical material 1 such that the polarization directions thereof are perpendicular to each other as shown in FIG. 8A, in a condition that the nonlinear optical material 1 allows the type 2 phase matching to two lights xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d, the sum frequency light xe2x80x9ccxe2x80x9d having a sum angular frequency (xcfx89S+xcfx89D) is outputted from the other face of the nonlinear optical material 1.
The phase matching refers to it that the velocity (phase velocity) of each incident light entered into the nonlinear optical material 1 and the velocity of harmonic light to the incident light like the sum frequency light excited by the incident light coincide with each other in the crystal of the nonlinear optical material 1.
Then, the type 2 phase matching refers to phase matching which is executed when the polarization directions of two incident lights are perpendicular to each other.
Meanwhile, the type 1 phase matching refers to phase matching which is executed when the polarization directions of two incident lights agree with each other.
The velocity of light advancing within the nonlinear optical material 1 differs depending upon the wavelength (frequency) and the advance direction to a crystalline axis.
Thus, for the velocity (phase velocity) of each incident light described above within the crystal and the velocity (phase velocity) of the sum frequency light within the crystal to coincide with each other, when the direction connecting an intersection between a refractivity ellipsoid of the incident light and a refractivity ellipsoid of the sum frequency light within the three-dimensional coordinates of the crystal is regarded as phase matching direction, the optical axis of each incident light described above is matched with the phase matching direction.
Further, the polarization direction of each incident light only has to be parallel to or perpendicular to the reference axis of a crystal existing within a plane at right angle to the phase matching direction.
More specifically, the nonlinear optical material 1 is cut out in the form of a rectangular pipe or cylinder having a plane perpendicular to that phase matching direction.
Currently, as such a nonlinear optical material 1, KTP (KH2, PO4), LN (LiNbO3), LT (LiTaO3), KN (KNbO3) and the like are available.
FIG. 9 is a block diagram showing the schematic structure of a conventional optical sampling waveform measuring apparatus including the nonlinear optical material 1 allowing the type 2 phase matching.
The measuring object light xe2x80x9caxe2x80x9d having the repetition frequency xe2x80x9cfxe2x80x9d of a pulse waveform under the angular frequency xcfx89D of light entered from outside is controlled in terms of its polarization direction to 90xc2x0 with respect to the reference direction (0xc2x0 direction) by a polarization direction controller 2 and after that, entered into a multiplexer 3.
On the other hand, a sampling light source 4 outputs the sampling light xe2x80x9cbxe2x80x9d having the repetition frequency (fxe2x88x92xcex94f) of a pulse waveform under the angular frequency xcfx89S different from the angular frequency xcfx89D of the aforementioned measuring object light xe2x80x9caxe2x80x9d.
As shown in FIG. 7B, the pulse width of this sampling light xe2x80x9cbxe2x80x9d is set by far narrower than the pulse width of the measuring object light xe2x80x9caxe2x80x9d.
After the polarization direction is controlled to for example, the reference direction (0xc2x0 direction) by means of a polarization direction controller 5, the sampling light xe2x80x9cbxe2x80x9d outputted from the sampling light source 4 is entered into the multiplexer 3.
The multiplexer 3 comprised of for example, a beam splitter (BS) allows the incident light to advance straight through a half mirror 3a and reflects it at right angle.
Therefore, the sampling light xe2x80x9cbxe2x80x9d having a polarization direction, which is the reference direction (0xc2x0 direction) and the measuring object light xe2x80x9caxe2x80x9d having a polarization direction which is at 90xc2x0 with respect to the reference direction (0xc2x0 direction) are entered into one face of the nonlinear optical material 1 which allows type 2 phase matching, disposed behind this multiplexer 3 and located on the optical axis of the sampling light xe2x80x9cbxe2x80x9d at the same time.
Consequently, a sum frequency light xe2x80x9ccxe2x80x9d having an angular frequency (xcfx89S+xcfx89D) is outputted from the other face of the nonlinear optical material 1 of type 2.
The sum frequency light xe2x80x9ccxe2x80x9d outputted from the nonlinear optical material 1 is entered into a light receiver 7 through an optical filter 6.
Light outputted from the nonlinear optical material 1 contains light (sum frequency light xe2x80x9ccxe2x80x9d) having the sum angular frequency (xcfx89S, xcfx89D) of the frequencies xcfx89S and xcfx89D of the aforementioned two lights xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d, lights having angular frequencies 2xcfx89S and 2xcfx89D which are twice the respective angular frequencies xcfx89S and xcfx89D although minute, and lights having the respective angular frequencies xcfx89S and xcfx89D which are not converted. Therefore, the components having these is angular frequencies 2xcfx89S, 2xcfx89D, xcfx89S and xcfx89D with the optical filter 6.
Then, the light receiver 7 converts the sum frequency light xe2x80x9ccxe2x80x9d to the electric signal xe2x80x9cdxe2x80x9d and transmits it to an electric signal processing system 8 on a next stage.
This electric signal processing system 8 creates an optical pulse xe2x80x9cexe2x80x9d of the measuring object light xe2x80x9caxe2x80x9d enlarged in time direction according to the above-described method from an electric signal xe2x80x9cdxe2x80x9d having the same waveform as the entered sum frequency light xe2x80x9ccxe2x80x9d shown in FIG. 7C and displays it on a display unit 9.
However, the conventional optical sampling waveform measuring apparatus shown in FIG. 9 has a following problem to be solved.
In order to improve the measuring accuracy of the optical pulse xe2x80x9cexe2x80x9d in the optical sampling waveform measuring apparatus for measuring the optical pulse waveform xe2x80x9cexe2x80x9d of the measuring object light using the sum frequency light xe2x80x9ccxe2x80x9d generated from the aforementioned nonlinear optical material 1, it is necessary to improve generation efficiency of the sum frequency light xe2x80x9ccxe2x80x9d generated from the nonlinear optical material 1 so as to improve the S/N ratio of the sum frequency light xe2x80x9ccxe2x80x9d.
More specifically, if the intensity of the sum frequency light xe2x80x9ccxe2x80x9d is assumed to be PSFG and the intensities of the measuring object light xe2x80x9caxe2x80x9d and the sampling light xe2x80x9cbxe2x80x9d are assumed to be PSIG, PSAM, the intensity PSFG of the sum frequency light xe2x80x9ccxe2x80x9d is expressed with a following expression:
PSFG=xcex7xc2x7RSIGxc2x7PSAM 
where xcex7 is a nonlinear conversion efficiency constant which is automatically determined depending on the kind and material of the nonlinear optical material 1 to be adopted.
As the nonlinear optical material 1 having a high nonlinear conversion efficiency constant xcex7, KTP, LN, KN and the like, which are the aforementioned inorganic nonlinear optical material 1, are employed.
However, to measure the optical pulse waveform within a single repetition cycle of the measuring object light having a repetition frequency of about several tens GHz, the S/N ratio of the sum frequency light needs to be 3 dB or more.
Thus, development of the nonlinear optical material 1 having a high nonlinear conversion efficiency constant xcex7 has been demanded.
As the nonlinear optical material which satisfies these demands, Jpn. Pat. Appln. KOKAI Publication No. 9-159536 has disclosed an optical sampling waveform measuring apparatus employing 2-adamantyl-5-nitropyridine (hereinafter referred to as AANP) which is not inorganic but organic nonlinear optical crystal.
Here, the nonlinear conversion efficiency constant xcex7 of the AANP, which is an organic nonlinear optical crystal, is on the order of 10xe2x88x922 which is by far higher than the nonlinear conversion efficiency constant xcex7 on the order of 10xe2x88x924 of KTP, LN, LT, KN and the like which are the aforementioned inorganic nonlinear optical materials.
Therefore, if the AANP which is an organic nonlinear optical crystal is employed for the optical sampling waveform measuring apparatus as the nonlinear optical material, the S/N ratio of the sum frequency light emitted from this AANP is improved, so that finally, the measuring accuracy for the optical pulse waveform is also improved.
However, even the optical sampling waveform measuring apparatus employing the AANP nonlinear optical crystal still has a following problem, which should be solved.
That is, a main measuring object light, which is subject to measurement with the optical sampling waveform measuring apparatus, is optical signal in an optical communication system.
Currently, the optical communication system has begun to use not only C band (1530 to 1565 nm) but also L band (1570 to 1610 nm) in order to aim at increase of transmission capacity.
Thus, a necessity of measuring optical signals in such a wavelength range of C band and L band occurs in the optical sampling waveform measuring apparatus also.
The AANP nonlinear optical crystal has such a problem that although its nonlinear conversion efficiency is high as described above, according to the conventional technology, the 3 dB band capable of generating the SFG light is about 40 nm, which is 1535 to 1575 nm (reported in ECOC ""96 ThB, 1.2) while no SFG light can be obtained in a wide band of 80 nm including the L band.
This reason is that if the measuring object wavelength changes from its initial condition even if the sampling wavelength is fixed, inconsistency in phase matching occurs rapidly so that the efficiency of conversion of the measuring object light and sampling light to the sum frequency light drops.
Because actually, the optical sampling waveform measuring apparatus defines the amplitude of a measurement value by the measuring object wavelength to be 3 dB, the measuring band based on the prior art is limited to 40 nm.
For the reason, if it is intended to measure a measuring object light in a band of 40 nm or more, it is necessary to prepare a multiplicity of the AANP nonlinear optical crystals corresponding to the band of the measuring object wavelength.
Further, the wavelength of the sampling pulse in the optical sampling waveform measuring apparatus needs to be switched between the C band and the L band.
For example, inconveniently, when measuring the C band, the sampling pulse wavelength is set to 1555 nm and when measuring the L band, it is set to 1590 nm and then, sampling is carried out through the AANP nonlinear optical crystal cut out corresponding to each wavelength.
Therefore, if it is intended to measure the C band and the L band, at least two optical sampling waveform measuring apparatuses need to be prepared corresponding to each band, which provides an important problem from viewpoint of cost performance.
The present invention has been achieved in views of the above-described problems and therefore, an object of the invention is to provide an optical sampling waveform measuring apparatus in which by applying a specific phase matching condition to the AANP which is a nonlinear optical crystal for generating a sum frequency light of the sampling light and the measuring object light, the 3 dB band width of the sum frequency light generation efficiency is increased twice or more so as to possess a measuring object band of 80 nm or more thereby achieving a wider band.
To achieve the above described object, according to a first aspect of the present invention, there is provided an optical sampling waveform measuring apparatus comprising:
a sampling light source (4) which emits a sampling light (b) having a single polarization direction whose pulse width is smaller than that of an inputted measuring object light (a) having a single polarization direction;
a multiplexer (3) which multiplexes the sampling light emitted from the sampling light source and the measuring object light on the same optical axis such that the polarization directions thereof are perpendicular to each other;
a nonlinear optical crystal (10) composed of 2-adamantyl-5-nitorpyridine (AANP) allowing type 2 phase matching to the sampling light and the measuring object light, which, when the sampling light and the measuring object light multiplexed by the multiplexer are entered, emits a sum frequency light (c) of the sampling light and the measuring object light, the sum frequency light having a sum angular frequency xcfx89D+xcfx89S, based on the measuring object light having an angular frequency xcfx89D and the sampling light having an angular frequency xcfx89S, with the polarization directions thereof being perpendicular to each other;
a light receiver (7) which converts the sum frequency light outputted from the nonlinear optical crystal to an electric signal (d);
a signal processing portion (8) which processes the electric signal outputted from the light receiver so as to display an optical pulse waveform (e) of the measuring object light; and
control means (2, 5) for, when the sum frequency light (c) of the sampling light and the measuring object light is emitted from the nonlinear optical crystal, controlling the polarization direction of the sampling light so as to be parallel to a predetermined reference axis located within a plane perpendicular to a phase matching direction (15) of the nonlinear optical crystal,
wherein the predetermined reference axis is a single axis maintaining parallelism with the crystalline axis of the nonlinear optical crystal even if the wavelength of inputted light is changed.
According to a second aspect of the present invention, there is provided an optical sampling waveform measuring apparatus according to the first aspect, wherein the sampling light source is capable of emitting plural sampling lights each having a different wavelength and any one of the plural sampling lights is selected and emitted.
According to a third aspect of the present invention, there is provided an optical sampling waveform measuring apparatus comprising:
a sampling light source (4) which emits a sampling light (b) having a single polarization direction whose pulse width is smaller than that of an inputted measuring object light (a) having a single polarization direction;
a multiplexer (3) which multiplexes the sampling light emitted from the sampling light source and the measuring object light on the same optical axis such that the polarization directions thereof are perpendicular to each other;
a nonlinear optical crystal (10) composed of 2-adamantyl-5-nitorpyridine (AANP) allowing type 2 phase matching to the sampling light and the measuring object light, which, when the sampling light and the measuring object light multiplexed by the multiplexer are entered, emits a sum frequency light (c) of the sampling light and the measuring object light, the sum frequency light having a sum angular frequency xcfx89D+xcfx89S, based on the measuring object light having an angular frequency xcfx89D and the sampling light having an angular frequency xcfx89S, with the polarization directions thereof being perpendicular to each other;
incident angle changing means (30) for changing an incident angle of each of the sampling light and measuring object light to be entered into the nonlinear optical crystal into the nonlinear optical crystal;
a light receiver (7) which converts the sum frequency light outputted from the nonlinear optical crystal to an electric signal (d);
a signal processing portion (8) which processes the electric signal outputted from the light receiver so as to display an optical pulse waveform (e) of the measuring object light; and
control means (2, 5) for, when the sum frequency light (c) of the sampling light and the measuring object light is emitted from the nonlinear optical crystal, controlling the polarization direction of the sampling light so as to be parallel to a predetermined reference axis located within a plane perpendicular to a phase matching direction (15) of the nonlinear optical crystal,
wherein the predetermined reference axis is a single axis maintaining parallelism with the crystalline axis of the nonlinear optical crystal even if the wavelength of inputted light is changed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.